There are many forms of electrical circuit protection including breakers and fuses but one attractive approach is limitation. Limitation, as opposed to breaking, involves the spontaneous increase of a pre-existing but negligible impedance which immediately limits the fault current to a defined and innocuous level. This can be achieved instantaneously with no delay, in contrast to breakers which have limited response times during which time the fault current may increase to the full value of the prospective asymmetric peak. FIG. 1 is a block diagram illustrating a typical circuit having a current limiter coupled in series to limit the amount of surged current flowing through the circuit, where current limiter 102 is to limit the amount of fault current drawn from AC source 101 by load 103. Breakers, when correctly designed and installed, will ultimately clear these currents but the faulted circuits and any included switch gear will inevitably be exposed to the fault current peak and RMS value prior to opening and clearing.
Present conventional technology surrounding fault current limitation can be divided into two categories: passive technology and active technology. Passive fault current limiters are inserted in series with the main circuit and are designed to always present impedance to the load current, regardless of the conditions of the load. The advantage of passive technology is that it is fail safe; there is a zero probability of failure and no detection circuitry is required to activate it. The disadvantage of passive technology is that it can be increase losses for the AC circuit, voltage regulation at the load, and possible large size and high cost. An example of passive technology used for fault current limitation is air cored reactors.
Active fault current limiters aim to be “smarter” in that they present almost zero impedance at less than normal load (nominal conditions) but change rapidly under the conditions of overload or fault conditions. The condition at which they change state is user defined. The advantage of active limiters is that they may be more efficient than passive limiters and smaller due to the fact that the limiting element is only inserted for short intervals. Their disadvantage is the complex detection circuitry that is needed to detect abnormal or overload conditions and force a change in state of the limiter. Often redundancy in main components is specified, causing added complication. A high component count, especially at higher voltages and currents leads to diminished reliability. This technology is classified as non fail safe as there is a non-zero probability of failure, the consequences of which can be costly. An example of active fault current technology is a static switch, using turn-off component technology (such as gate turn-off thyristor (GTO), insulated gate bipolar transistor (IGBT), or integrated gate-commutated thyristor (IGCT)).